Local pressure sensing in a plasma processing system

ABSTRACT

A plasma processing system includes a process chamber, a source configured to generate a plasma in the process chamber, a platen configured to support a workpiece in the process chamber, and a pressure sensor positioned adjacent to the workpiece. The pressure sensor is configured to monitor a local pressure adjacent to the workpiece. A method includes generating a plasma in a process chamber, supporting a workpiece in the process chamber, and monitoring a local pressure adjacent to the workpiece with a pressure sensor positioned adjacent to the workpiece.

FIELD

This disclosure relates to workpiece processing systems, and moreparticularly to outgassing rate detection in a workpiece processingsystem.

BACKGROUND

A workpiece processing system may include, but not be limited to, plasmaprocessing systems such as doping, etching, and deposition systems. Aworkpiece processing system may also include a beam-line doping systemsuch as a beam-line ion implanter. As a workpiece in such systems istreated, outgassing may occur from the workpiece. Such outgassing canlead to unstable and/or non-repeatable conditions. Therefore, it isdesirable to sense and control such outgassing.

For instance, two types of workpiece processing systems include plasmadoping and beam-line ion implanters. In a plasma doping ion implanter, asource may generate plasma within a process chamber. A platen ispositioned in the process chamber for supporting a workpiece, and ionsmay be accelerated from the plasma into the wafer. In a beam-line ionimplanter, a desired impurity material is ionized in an ion source, theions are accelerated to form an ion beam of prescribed energy, and theion beam is directed at a front surface of the workpiece.

In both the plasma doping and beam-line ion implanters, it may bedesirable to operate the ion implanter at a relatively high dose rate inorder to increase throughput. However, operating at such relatively highdose rates can exacerbate outgassing from the workpiece, e.g., asemiconductor wafer or wafer in one instance. Outgassing from the wafermay occur when ions strike films or layers on the wafer such as aphotoresist layer. A photoresist layer is used to mask selected areas ofthe wafer surface so ions are implanted only in the unmasked areas.During ion implantation, the energetic ions may break up chemical bondswithin the photoresist layer. As a result, outgassing byproducts such asvolatile organic chemicals and/or other particles may be released. Thismay be referred to generally in the art as “outgassing,” or “photoresistoutgassing” when the outgassing is attributable to the photoresistlayer.

High rates of outgassing from the wafer in ion implanters can lead tounstable and/or non-repeatable implant conditions. High rates ofoutgassing can also contribute to contamination in ion implanters as theenergetic ions collide with the outgassing byproducts. In addition, inplasma doping ion implanters, outgassing byproducts can lead to arcingin the process chamber that can damage the devices being formed on thewafer. Therefore, it is desirable to sense a parameter representative ofan outgassing rate. One conventional parameter that may be sensed inplasma doping ion implanters is global pressure in the process chambersensed by a pressure sensor positioned relatively far away from thewafer. However, this pressure sensor has accuracy and time delaydrawbacks.

Accordingly, there is a need to provide another technique for outgassingrate detection that overcomes the above-described inadequacies andshortcomings.

SUMMARY

According to a first aspect of the disclosure, a workpiece processingsystem is provided. The workpiece processing system includes a platenconfigured to support a workpiece, a source configured to provide anelectromagnetic wave proximate a front surface of the workpiece, and adetector configured to receive at least a portion of the electromagneticwave and provide a detection signal representative of an outgassing ratefrom the workpiece of outgassing byproducts.

According to another aspect of the disclosure, a method of detectingoutgassing is provided. The method includes providing an electromagneticwave proximate a front surface of a workpiece, receiving at least aportion of the electromagnetic wave, and providing a detection signalrepresentative of an outgassing rate from the workpiece of outgassingbyproducts.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present disclosure, reference is madeto the accompanying drawings, in which like elements are referenced withlike numerals, and in which:

FIG. 1 is a block diagram of a workpiece processing system consistentwith the disclosure;

FIG. 2 is a block diagram of one embodiment of a plasma doping ionimplanter having a light source and light detector positioned within aprocess chamber;

FIG. 3 is a block diagram of another embodiment of a plasma doping ionimplanter having a light source and light detector positioned externalto the process chamber;

FIG. 4 is a plan view of a semiconductor wafer that may be treated inthe systems of FIGS. 1-3;

FIG. 5 is a cross sectional view of the semiconductor wafer of FIG. 4;and

FIG. 6 illustrates plots of a detection signal versus time and anoutgassing rate and dose rate over a similar time period.

DETAILED DESCRIPTION

FIG. 1 is a block diagram of a workpiece processing system 100consistent with an embodiment of the disclosure. The workpieceprocessing system 100 may include, but not be limited to, dopingsystems, etching systems, and deposition systems. Doping systems mayinclude a plasma doping ion implanter or a beam-line ion implanter. Theworkpiece processing system 100 includes a platen 114 to support aworkpiece 120. The workpiece processing system 100 also includes asource 160, a detector 162, a controller 146, a user interface system148, and rate components 151. The source 160 is configured to provide anelectromagnetic wave 165 proximate a front surface 122 of the workpiece120. The source 160 may be a light source to emit a light wave or amicrowave source to emit a microwave. The light source may be a laser, alight-emitting diode, or other light source known in the art. The lightwave may have a frequency in the ultraviolet spectrum, the visiblespectrum, and the infrared spectrum. The light source may provide apulsed or continuous light wave. The microwave may have a frequency inthe microwave spectrum. The detector 160 receives at least a portion ofthe electromagnetic wave 165 depending on the outgassing rate ofoutgassing byproducts 190 from the workpiece 120. The source 160 may bepositioned at one end of the workpiece 120, while the detector 162 maybe positioned on an opposing end of the workpiece 120. The detector maybe a light detector or a microwave detector. The light detector may by aphotodiode, a photoresistor, or other light detector known in the art.The light detector may also include a collimator to redirect receivedlight waves to the same area of the light detector.

The controller 146 can be or include a general-purpose computer ornetwork of general-purpose computers that may be programmed to performdesired input/output functions. The controller 146 can also includeother electronic circuitry or components, such as application specificintegrated circuits, other hardwired or programmable electronic devices,discrete element circuits, etc. The controller 146 may also includecommunication devices, data storage devices, and software. The userinterface system 148 may include, but not be limited to, devices such astouch screens, keyboards, user pointing devices, displays, printers,etc. to allow a user to input commands and/or data and/or to monitor theworkpiece processing system 100 via the controller 146.

The rate components 151 can include differing components known in theart to control a rate of particles 141 directed to the workpiece 120.When the workpiece processing system 100 is an ion implanter, the ratecomponents 151 can include dose rate components to control the rate ofions directed to the workpiece 120. For a beam-line ion implanter, thedose rate components may include an ion source, focusing elements suchas lenses, a mass resolving slit, a scanner, and/or other beam linecomponents known in the art. The dose rate components in a beam-line ionimplanter may change the quantity of ions/second generated and/or reducea cross sectional size of the ion beam having the same number ofions/second generated since dose rate may be given by ions/second/cm².For plasma doping ion implanters, the dose rate components may include asource to change the plasma density, a biasing source to change the rateat which ions are accelerated from the plasma towards the wafer, and/orother components known to those skilled in the art.

In operation, the workpiece processing system 100 may direct particles141 towards the workpiece 120. The particles 141 may break up chemicalbonds within a layer, e.g., a photoresist layer 121, of the workpiece120. As a result, outgassing byproducts 190 such as volatile organicchemicals and/or other particles may be released from the workpiece 120.The outgassing byproducts 190 may include photoresist outgassingbyproducts released from the photoresist layer 121. The photoresistbyproducts may include, but not be limited to, carbon compounds, watervapor, hydrogen, and nitrogen.

The electromagnetic wave 165 may be partially deflected and/or absorbedby the released outgassing byproducts 190. The amount of theelectromagnetic wave deflected and/or absorbed depends on the outgassingrate of the outgassing byproducts 190. Therefore, the intensity of thereceived electromagnetic wave by the detector 162 may be reducedcompared to a nominal intensity in the presence of no outgassing. Thecontroller 146 may determine the amount of reduction and correlate thesame to an outgassing rate.

The source 160, the detector 162, and the frequency of theelectromagnetic wave 165 may be selected to maximize deflection and/orabsorption by the expected outgassing byproducts 190. In one embodiment,the expected outgassing byproducts 190 may include carbon compounds suchas CH₄. In this instance, the source 160 may be a light source and theelectromagnetic wave 165 may be a light wave having a frequency in theinfrared frequency spectrum. The detector 162 may be a light detectorand the detection signal is representative of an outgassing rate of thecarbon compounds. In one specific example, the wavelength of the lightwave was selected at 3.392 microns to detect CH₄.

In another embodiment, the expected outgassing byproducts 190 mayinclude water vapor. In this instance, the source 160 may be a microwavesource and the electromagnetic wave 165 may be a microwave. The detector162 may be a microwave detector and the detection signal isrepresentative of an outgassing rate of the water vapor. In one specificexample, the wavelength of the microwave was selected at 12 centimeters(2.45 GHz frequency) to detect water vapor.

In yet another embodiment, the expected outgassing byproducts 190 mayinclude hydrogen. In this instance, the source 160 may be a light sourceand the electromagnetic wave 165 may be a light wave having a frequencyin the ultraviolet frequency spectrum. The detector 162 may be a lightdetector and the detection signal is representative of an outgassingrate of the hydrogen. In one specific example, the wavelength of thelight wave was selected at 1216 angstroms to detect hydrogen atoms.

The frequency range of the electromagnetic 165 wave may be selected tomaximize deflection and/or absorption by the expected outgassingbyproducts 190. The detector 162 may therefore sense a decrease in theintensity of the received electromagnetic wave depending on theoutgassing rate of the outgassing byproducts 190 and provide a detectionsignal to the controller 146. The controller 146 may adjust the rate ofthe particles 141 in response to the detection signal. For example, inan ion implanter the controller 146 may lower a dose rate of ionsdirected towards the workpiece 120 in response to an excessiveoutgassing rate and increase the dose rate to a maximum level inresponse to a modest outgassing rate.

FIG. 2 is a block diagram of one embodiment of a plasma doping ionimplanter 200 consistent with the workpiece processing system 100 ofFIG. 1 where the source 160 is a light source 260, the detector 162 is alight detector 262 and the electromagnetic wave 165 is a light wave 265.The plasma doping ion implanter 200 may include a process chamber 210defining an enclosed volume 212. In this embodiment, the light source260 and light detector 262 may be positioned within the process chamber210. The platen 114 may be positioned within the process chamber 210 toprovide a holding surface for holding the workpiece 120. In oneinstance, the workpiece 120 may be a semiconductor wafer having a diskshape. The workpiece 120 may, for example, be clamped to a flat surfaceof the platen 114 by electrostatic or mechanical forces. In oneembodiment, the platen 114 may include conductive pins (not shown) forconnection to the workpiece 120.

There are differing methods to generate a plasma 240 within the processchamber 210. In one embodiment, a source 230 may cooperate with theanode 224 to generate the plasma 240. The source 230 may be a highvoltage power source to provide high voltage pulses to the anode 224. Inother embodiments, the source 230 may be a RF source such as an RF powersupply to supply RF power to one or more antennas (not illustrated) togenerate the plasma 240 in the process chamber 210. Other sources andconfigurations for generating the plasma 240 in the process chamber 210will be known to those skilled in the art. The plasma 240 may include aplasma sheath 242 in the region between the plasma 240 and the workpiece120.

The plasma doping ion implanter 200 may also include a bias source 272electrically coupled to the platen 114 to bias the platen 114 toaccelerate ions from the plasma 240 into the workpiece 120. The biassource 272 may be a DC pulsed power supply, an RF power supply, or otherpower supply known by those skilled in the art. When the bias source 272is a DC pulsed power supply, the duty factor may be selected to providea desired dose rate. A negative voltage pulse would accelerate positiveions from the plasma 240 towards the wafer 120. The enclosed volume 212of the process chamber 210 may be coupled thought a controllable valve232 to a vacuum pump 234. A gas source 236 may be coupled through a massflow controller 238 to the chamber 210.

A shield ring 266 may be disposed around the platen 114. As is known inthe art, the shield ring 266 may be biased to improve the uniformity ofimplanted ion distribution near the edge of the workpiece 120. Faradaysensors such as Faraday cups 250, 252 may also be positioned in anassociated recess of the shield ring 266. The Faraday sensors monitorion current and may input a signal to the controller 146 representativeof the dose rate. The controller 146 may process a signal from theFaraday sensors 250, 252 as well as the light detection signal from thelight detector 262 to determine ion dose. For clarity of illustration,the controller 146 is illustrated as providing only an output signal tothe bias source 172. Those skilled in the art will recognize that thecontroller 146 may provide output signals to other components of theplasma doping ion implanter 200 and receive input signals from the same.

In operation, the gas source 236 supplies an ionizable gas containing adesired dopant for implantation into the workpiece 120. Examples ofionizable gas include, but are not limited to, BF₃, N₂, Ar, PH₃, AsH₃,B₂H₆, H₂, Xe, SIH₄, SIF₄, GeH₄, GeF₄, CH₄, CF₄, AsF₅, PF₃, and PF₅. Themass flow controller 238 regulates the rate at which gas is supplied tothe process chamber 210. The source 230 may generate the plasma 240within the process chamber 210, and the bias source 272 may bias theplaten 114 to accelerate ions from the plasma 240 into the workpiece120. The energetic ions may cause outgassing byproducts 190 to bereleased from the wafer. The outgassing byproducts may include, but notbe limited to, carbon compounds, water vapor, hydrogen, and nitrogen.The released outgassing byproducts 190 may absorb and/or deflect atleast a portion of the light wave 265 provided by the light source 260.The light source 260 may provide a pulsed or continuous light wave 265.A pulsed light wave permits discrimination versus plasma generatedlight. The pulsed light wave may be pulsed synchronously with the biassignal provided by the bias source 272 or at higher sampling multiples.The pulsed light wave may therefore improve the signal to noise ratio ofthe portion of the received light wave.

Regardless of whether the light source 260 provides a pulsed orcontinuous light wave 265, the light detector 262 may sense a decreasein the intensity of the received light wave 265 depending on theoutgassing rate of the outgassing byproducts 190 and provide a detectionsignal to the controller 146 representative of the same. The controller146 may compare the actual intensity of the received light signal with adesired or nominal intensity and control the dose rate of the implanter200 in response thereto. The controller 146 may control the dose rate inone embodiment by controlling the duty factor of a pulsed DC voltagesignal provided to the platen 114, e.g., by the bias source 272.

FIG. 3 is a block diagram of another embodiment of a plasma doping ionimplanter 300 illustrating the light source 260 and light detector 262positioned external to the process chamber 210. Selected components ofthe implanter 300 similar to FIG. 2 have been omitted from the drawingfor clarity. A first window 302 is positioned relative to the lightsource to allow the light wave 265 from the light source to pass. Asecond window 304 is positioned relative to the light detector 262 toallow the light wave 265, or a portion thereof depending on the amountof light absorbed and/or deflected by the outgassing byproducts 190, topass. The windows 302, 304 may be fabricated of quartz, glass, or someother transparent material known in the art. Positioning the lightsource 260 and light detector 262 external to the process chamber 210protects them from adverse conditions within the interior volume 212defined by the process chamber 210.

It is possible that over time the first and second windows 302, 304 maybecome partially opaque due to particle deposits forming thereon. Thismay reduce transmission of an electromagnetic wave such as a light wave.Such window coating may be compensated for by normalizing the lightlevel received prior to processing the workpiece. For example, the lightsource 260 may provide a light wave prior to processing and the lightdetector 262 may quantify the intensity of the received light wave giventhe current conditions of the windows 302, 304. The ratio of thereceived signal just prior to processing with the received signal duringprocessing can be utilized to compensate for deposits on the windows302, 304.

Although FIGS. 2 and 3 illustrate, among other things, the light source260 and light detector 262 positions relative to the process chamber 210of plasma doping ion implanters, the light source 260 and light detector262 could also be similarly positioned relative to another chamber suchas an end station chamber of a different plasma processing chamber or abeam-line ion implanter.

FIG. 4 is a plan view of a semiconductor wafer 420 having a disk shapeand a diameter D1. FIG. 5 is a cross sectional view of the semiconductorwafer of FIG. 4. The semiconductor wafer 420 may have a front surface422 defining a plane 502. The light wave 265 from the light source maybe generally directed along a path 504 parallel to the plane 502. Thepath 504 may be a distance (x) from the plane 502. In one embodiment,the distance x may be less than or equal to 10% of the diameter D1 ofthe wafer 420. In this way, the light beam 265 is positioned on a pathproximate the front surface 422 of the semiconductor wafer 420 tointercept outgassing byproducts 190 released from the same. For example,with a 300 mm diameter semiconductor wafer, the light beam 265 may bepositioned a distance (x) from the semiconductor wafer of less than orequal to 30 mm. This positioning also enables the light wave to passthrough the plasma sheath 242 in a plasma doping ion implanter.

FIG. 6 illustrates a plot 602 of a detection signal that may be providedby the detector 162 over particular time period. A plot 604 of anoutgassing rate of the outgassing byproducts 190 and a plot 606 of doserate for an ion implanter over the same time period are alsoillustrated. In one embodiment, the detection signal may be a voltagesignal. The voltage signal may have a maximum voltage level (V2)indicating that substantially no portion of the electromagnetic wave 165is absorbed and/or deflected by any outgassing byproducts 190 betweentimes t0 and t1. Hence the corresponding outgassing rate is effectivelyzero over the same time period and the dose rate may be at a maximumdose rate (dr2).

As illustrated by plot 602, the detection signal may be at a desiredvoltage level (V2) and then decrease until it reaches a threshold level(V1) at time t2. As illustrated by plot 604, the decrease in thedetection signal is representative an increase in the outgassing ratefrom the wafer 120 over the same time period. The threshold voltagelevel (V1) may be selected to be associated with a high outgassing rate(og2). In response, the controller 146 may control the dose rate of animplanter by reducing the dose rate from an initial maximum dose rate(dr2) to a comparatively lower dose rate (dr1) at time t2. In oneembodiment, the controller 146 may control the dose rate by controllingthe duty factor of a pulsed DC voltage signal provided to the platen114, e.g., by the bias source 272. In one example, the duty factor maybe lowered by about 20% to 30% from its initial value at the maximumdose rate (dr2). The controller 146 may then maintain the dose rate atthe lower dose rate (dr1) for a particular delay period or until time t3to give the outgassing time to dissipate relative to the high outgassingrate (og2). The controller 146 may then start to ramp up the dose rateback to its maximum value (dr2) at time t4.

Advantageously, the detector 162 receives at least a portion of theelectromagnetic wave 164 that is representative of an outgassing rate ofthe outgassing byproducts 190. In an ion implanter, the dose rate canthen be modulated in response to the outgassing rate. This enablesimprovements in throughput as the dose rate can be maximized as long asthe detection signal is indicative of a relatively lower outgassingrate. In addition, the dose rate can be lowered if the detection signalis indicative of a relatively higher outgassing rate to control problemssuch as contamination and dose repeatability issues. Lowering the doserate in a plasma doping implanter can also control arcing problems.

The present disclosure is not to be limited in scope by the specificembodiments described herein. Indeed, other various embodiments of andmodifications to the present disclosure, in addition to those describedherein, will be apparent to those of ordinary skill in the art from theforegoing description and accompanying drawings. Thus, such otherembodiments and modifications are intended to fall within the scope ofthe present disclosure. Further, although the present disclosure hasbeen described herein in the context of a particular implementation in aparticular environment for a particular purpose, those of ordinary skillin the art will recognize that its usefulness is not limited thereto andthat the present disclosure may be beneficially implemented in anynumber of environments for any number of purposes. Accordingly, theclaims set forth below should be construed in view of the full breadthand spirit of the present disclosure as described herein.

1. A plasma processing system comprising: a process chamber; a source configured to generate a plasma in the process chamber; a platen configured to support a workpiece in the process chamber; a gas pressure sensor positioned adjacent to the workpiece, the gas pressure sensor configured to monitor a local pressure adjacent to the workpiece; a bias source configured to bias the platen to accelerate ions from the plasma into the workpiece; and a shield ring disposed around the platen, wherein the pressure sensor is positioned within the shield ring and is configured to monitor the local pressure during the acceleration of ions from the plasma into the workpiece.
 2. The plasma processing system of claim 1, wherein the shield ring has an interior cavity and an opening from the interior cavity to a location on an exterior surface of the shield ring, wherein the gas pressure sensor is positioned within the interior cavity.
 3. The plasma processing system of claim 1, wherein the shield ring has an interior cavity, a first opening from the interior cavity to a first location on an exterior surface of the shield ring, and a second opening from the interior cavity to a second location on the exterior surface of the shield ring, wherein the gas pressure sensor is positioned within the interior cavity.
 4. The plasma processing system of claim 1, wherein a Faraday sensor is positioned in a recess of the shield ring, and wherein the shield ring has an interior cavity and an opening from the interior cavity to a location proximate the Faraday sensor, wherein the gas pressure sensor is positioned within the interior cavity.
 5. The plasma processing system of claim 1, further comprising a controller configured to receive a sensor signal from the gas pressure sensor representative of the local pressure, the controller further configured to compare the local pressure with a desired pressure, wherein the comparison is representative of an outgassing rate from the workpiece.
 6. The plasma processing system of claim 5, wherein the controller is further configured to control a dose rate of the ions by controlling the bias source in response to the comparison of the local pressure to the desired pressure.
 7. The plasma processing system of claim 6, wherein the controller is configured to lower the dose rate from a maximum dose rate if the local pressure exceeds a threshold pressure.
 8. The plasma processing system of claim 7, wherein the threshold pressure is at least 20% greater than the desired pressure.
 9. The plasma processing system of claim 1, wherein the gas pressure sensor is configured to monitor pressure within a pressure range from about 0.1 to 50 millitorr.
 10. The plasma processing system of claim 1, wherein the gas pressure sensor has a response time less than about 10 microseconds. 